Overmodulation alarm



0a. 12, 1948. J. E. SMITH 2,451,361

OVERMODULAT I ON ALARM Filed March 31, 1944 INPUT SOURCE INVEN TOR.JAMES ERNEST SMITH.

ATTORNEY.

Patented Got. 12, 1948 UNITED OVERMODULATION ALARM James Ernest Smith,Jackson Heights, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application March 31, 1944, Serial No. 529,007

4 Claims. 1

This invention relates to a new and useful simple type of audible orvisible overmodulation alarm indicator system, and is particularlyadapted to indicate an overmodulation condition of ultra high frequencytransmitters.

An object of this invention is to simplify and improve audible orvisible overmodulation alarm indicators.

Another object of this invention is to provide an improved modulationsystem for use with an ultra high frequency remote control transmitter.

A feature of this invention is the arrangement of an alarm system inwhich a pair of gas filled triodes are connected to the modulationcircuit such that one tube is normally conductive and the other tube isnon-conductive, and in case of overmodulation the conductive state ofthe tubes is altered by a change of the bias voltage which in turn stopsthe normal conduction of one of the tubes and causes the other tube toconduct. At the same time an amplifier stage operates to ac tuate thealarm.

In any modulated signal, the modulation should not be over 100% as it iswell known that if the carrier is overmodulated by more than 100% themodulated wave will then be distorted.

The purpose of this invention is to sound an alarm to indicateimmediately any overmodulation condition of the signal carrier.

This invention will best be understood by referring to the accompanyingdrawing in which the direct current modulated envelope is connected tothe outer ends of a resistance or potentiometer R2 having a variable tapwhich is connected to ground. The upper end of resistance R2 isconnected to a fixed resistor R1 which is connected in series with thegrid of a gas filled triode V1. The lower end of resistance R2 isconnected to the positive side of a bias battery B1. The negative sideof battery B1 is connected to a fixed resistor R3 which connects to thegrid of a second gas filled triode V2. The gas filled triodes V1 and V2are of the trigger action type known in the trade as Thyratrons. Thesegas filled tubes are constructed so that they do not conduct a currentuntil a certain critical potential is obtained. A suflicient negativecharge on the grid prevents the electron fiow from the cathode to theanode, so as to maintain a high impedance in the anode cathode circuit,but when the negative charge on the grid is altered by a change ofcharge, such as a positive charge, the electron fiow from cathode toanode reduces the tube impedance such that the positive charge thenplaced on the anode causes current to flow from-anode 2 to cathoderegardless of the then potential of the grid. Therefore, it will be seenthat the grid only controls the starting of the current flow through theanode cathode circuit.

The anodes of the tubes V1 and V2 are connected together in parallel tothe positive side of a voltage supply or battery B2. The negative side.of the battery B2 is connected to ground. The cathode of the gas filledthyratron V2 connects to one terminal of commutating condenser O, to thevariable arm of potentiometer P, and to ground through resistor R5. Thecathode of thyratron V1 connects to the other terminal of commutatingcondenser O, to the center tap of transformer T1 through resistor R4,thence to ground and the cathodes of amplified tubes V3 and V4 throughresistor R6 and bias battery B3. It. will be seen that transformers T1and T2 together with amplifier tubes V3 and V4 form a conventionalpush-pull amplifier. The input source F is connected to the primary ofinput transformer T1. The secondary of input transformer T1 has itsouter ends connected to the grids of the amplified tubes V3 and V4. Theanodes of amplifier tubes V3 and V4 are connected to the primary ofoutput transformer T2. The potentiometer P is connected across theprimary winding of output transformer T2.

In the normal operation of the device, that is, when the modulation ofthe direct current modulating envelope is not more than the gas filledtriode V1 is conducting and triode V2 is cut off or non-conducting dueto the biasing voltage of battery B1. The current flowing through V1,resistor R4 and resistor R6 produces an IR. drop in the resistor R6 suchthat the grids of the amplifier tubes would be maintained positive withrespect to their cathodes if it were not for the bias potential ofbattery B3. The value of R6 is chosen so that the grids are stillmaintained below cutoff and thus nocurrent flows through tubes V3 andV4. Now when the modulation current flowing through resistor R2 exceeds100% or I a predetermined level, depending upon the value of B1,suflicient potential will be developed across R2 to overcome the normalbias on the battery B1 thus causing gas triode V2 to strike or conduct.At the same instant condenser O (which now has a voltage across it equalto the potential of battery B2 less the drop in tube V1) discharges,which extinguishes tube V1 and increases the IR drop in Rs thus raisingthe grids of V3 and V4 above cutoff and allowing the amplifier tubes toconduct and operate as a class A amplifier. The output of this amplifiercan then actuate any 3 is suitable alarm device such as a standardbuzzer or bell. If desired, the bell could as well be replaced with arelay to operate any desired alarm indicator. The alarm will continue tosound so long as the modulation current exceeds 100%,

because the amplified tube bias permits a current flow through theoutput transformer T2. When the modulation current returns to normal,the current through R2 drops and the grids of amplifier tubes Va and V4are again biased below cut ofi and the alarm stops as the current thenceases to fiow in the amplified output transformer T2. At the same timethe discharge from condenser O extinguishes tube V2 and causes tube V1to conduct again. r

While I have indicated and described a system of my invention, it willbe apparent to one skilled in the art that my invention is by no meanslimited to the particular system shown and described, but that manymodifications may be made without departing from the scope of myinvention.

What -I-clalm is: V a

1. In an overmodulation indicator, a source of modulated energy, anamplifier stage tube having a control grid and cathode coupled to asource cf voltage and having an anode and cathode coupled to anindicator, a resistor wherein a potential drop is developed whichchanges when the'magnitude of the modulated energy changes, a first gastube of the control grid type having output electrodes in circuit with aresistor for biasing the control grid of said amplifier, the arrangementbeing such that when the gas tube is conductive the amplifier tube isbiased to cut cit, a coupling between the control grid of said first gastube and said first resistor such that said gas tube is conductive inthe absence of overmodulation of said energy, a second gas'tube of thecontrol grid type having output electrodes including a resistor in apotential-supply circuit for the anode of said amplifier, thearrangement being such that the potential on said anode is low wher'isaid last nan ied gas tube is nonconductive, a coupling between thecontrol grid of the last nained gas tube and said first resistorsuchthat said last named gas tube is non-conductive in the absence ofoverrriodulation of said energy, and a charging capacitor intercouplingsaid gas tubes to helpekting'uishsaid first gas tubewhen said second gastube is made conductive.

2.111 apparatus for producing indications of alterations of analternating current voltage whichexceeds a predetermined magnitude incombination, a resistor across which said voltages are impressed; twoelectron discharge devices of the gaseous type each having electrodesincluding an anode, cathode and control-grid; an amplifier stageincluding an electron discharge device having'arcontrol grid and havingan input excited by energy and having an anodelcoupled .toan alarmto beoperated by amplified energy; a biasing circuit for said last namedcontrol grid including a resistor in the output circuit of one gas tube;a potential supply circuit for said last namedanode including a resistorin the output circuit of the other gas tube; a storage condenser coupledto said resistors; means including a connection-between said first namedresistor and the control grid of said one gas tube for biasing saidonegastube to be conductive when said alternating current is of amagnitude less than said predetermined magnitude; and means including aconnectionbetween the said first named resistor '4 current is of amagnitude less than said predetermined magnitude.

3. In a device of the class described, a source of modulated energy, anamplifier stage tube having a control grid and cathode coupled to asource of voltage and having an anode and cathode coupled to an alarmdevice, a resistor wherein a potential drop is developed which changeswhen the magnitude of the modulated energy changes, a,first gas tube ofthe control grid type having output electrodes in circuit with aresistor for biasing the control grid of said amplifier, the arrangementbeing such that when the gas tube is conductive the amplifier tube isbiased to out off, stee ing between the control grid of said first gastube and said first resistor such that said gas tube is conductive inthe absence of overmedulation of said energy, a second gas tube of thecontrol grid type having output electrodes including a resistor in apotential supply circuit for the anode of said amplifier, thearrangement being such that the potential on said anode is low whensaidlast named gas tube is nonconductive, a coupling between the controlgrid of the last named gas tube and said first resistor such that saidlast named gas tube is nonconductive in the absence of overmodulation ofsaid energy, and a charging capacitor intercoupling said gas tubes andarranged to discharge when the source of modulated energy isovermodulated and to help extinguish said first gas tube when saidsecond gas tube. is made conductive, whereby a change of bias is made inthe control grid of the amplifier and permits its output circuit tobecome active and actuate said alarm device.

4. In an overmodulation alarm device, a source of 'modulated energy; aresistance connected across said source of modulated energy having avariable tap connected to ground; a push-pull amplifier having a pair ofelectron discharge devices, each electron discharge device having acontrol grid, an anode, and a cathode; a first gas filled tube having atleast a grid, anode and cathode, said first gas filled tube having itsgrid in series with a second resistor and one end of said firstmentioned resistor and its grid normally biased to conduct; a second gasfilled tube having at least a grid, anode and cathode, said second gasfilled tube havingits grid normally biased to be nonconducting andhaving its anode connected to the anode of said first mentionedgas-filled tube; a first source of bias potential connected at one endthrough a third resistor to the grid of said second gas filled] tube andthe other end to the other end of said first mentioned resistor, asecond source of bias potential connected at one end to ground and tothe cathodes of said electron discharge devices, the other end of saidsecond source of bias potential being connected through a fourthresistor to the center connection of an input transformer, Winding, saidinput transformer winding having ends connected to the grids of saidelectron discharge devices; a

condenser having its terminals connected between the cathodes of saidfirst and second mentioned gas filled tubes; a fifth resistor having oneterminal connected to the cathode of said second gas filled tube' theother terminal of said fifth resistor being connected to ground and oneside of a source of potential for said anodes of said gas filled tubes;a sixth resistance connected betweenthe cathode of said first gas filledtube and a junction point of said fourth resistor and the centerconnection of said input transformer Winding to normally bias theelectron discharge devices below cut off to prevent conductiontherethrough; an output transformer, including primary and secondarywindings, having its primary winding connected to the anodes of saidelectron discharge device; an alarm device connected in circuit with thesecondary of said output transformer; a potentiometer across the primaryof said output transformer and a connection from a tap on saidpotentiometer to the cathode of said second gas filled tube, and oneside of said fifth resistance, whereby the grids of said electrondischarge devices of said push-pull amplifier are normally biased belowcut off when there is normal modulation in said source of modulatedenergy, and the electron discharge devices are made conductive tooperate said alarm device when there is overmodulation in said source ofmodulated energy.

- J. ERNEST SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

